The DSL technology uses existing copper wire, which was originally deployed for telephone service, to provide broadband service without large investment. Hence the DSL has been deployed around the world. In an Asymmetric Digital Subscriber Line (ADSL) technology, the data rate in the downlink direction from the Central Office (CO) to the Customer Premises Equipments (CPE) is much higher than that in the uplink direction from the CPE to the CO, which is suitable for residential broadband access because the data rate of ADSL technology is usually asymmetric. Therefore, the ADSL technology has the technological advantage in the residential broadband access application.
ADSL uses higher frequency than the plain old telephone service(POTS) does, the frequency band of the ADSL (here ADSL is the first generation ADSL) and ADSL2 (the second generation ADSL) is 25.875 KHz˜1104 KHz, and the frequency band of ADSL2+ is 25.875 KHz˜2208 KHz, while POTS uses a frequency band of 0˜4 KHz. The ADSL and the POTS may be transmitted on a same telephone line simultaneously by using frequency division multiplex (FDM) technology. The ADSL and the POTS can be divided and mixed by transceivers in a Splitter at both sides, since the ADSL and the POTS occupy different frequency bands. The DSL Access Multiplexer (DSLAM) is a device which provides aggregation for multiple ADSL line and interfaces to the broadband network. Along with the development of silicon technologies, the port density of DSLAM keeps rising; one rack generally provides more than 2000 ADSL access ports. Since the ADSL makes use of the Multi-Carrier Modulation (MCM) technology, the modulated signal has a high Peak to Average Ratio (PAR) which requires the ADSL line driver circuit to use a high voltage power supply. Compared with the ADSL transmitting power level, the power consumed in the ADSL line driver circuit is much higher because of the poor efficiency of ADSL line driver circuit. FIG. 1 illustrates the correspondence relationship between the ADSL transmitting power level by using class AB power amplifier and corresponding power consumption of the ADSL line driver circuit. The horizontal axis represents the ADSL transmitting power level, and the vertical axis represents the power consumed by the ADSL line driver circuit. It can be seen that, the bigger the ADSL transmitting power level is, the bigger the power consumed by the ADSL line driver circuit is. Along with the augmentation of ADSL transmitting power level, the value for the power consumed by the ADSL line driver circuit exceeding the ADSL transmitting power level keeps rising, for example, when the ADSL transmitting power level is 20 dBm, i.e. 100 mW, the power consumed by the driver circuit attains 700˜800 mW.
Since the ADSL mainly operates in the limiting rate mode and the signal-noise-ratio introduced by the power consumed by the ADSL line driver circuit usually exceeds the practical requirement greatly. The noise margin often attains more than 30 dB. In practice, most of the powers consumed by the ADSL line driver circuit are wasted, the heat introduced by consuming powers will shorten the life of device, and the crosstalk to the adjacent line pairs will impact the operation of other services, which may reduce the penetration rate (ratio of number of pair in a bundle which can carry ADSL service versus the total number of pair in a bundle) of ADSL in a bundle.
To reduce the ADSL transmitting power level, in the ADSL standard G.992.1, the International Telecommunications Union-Telecommunication Standardization Sector (ITU-T) defines the gain scaling range of the ADSL transmitting power level as −14.5˜+2.5 dB, the recommended gain scaling range is −2.5 dB˜+2.5 dB. The CO device adjusts the ADSL transmitting power level according to the gain scaling value proposed by the remote device. The disadvantage of the method is that the range of the power reduction is limited. In the method, the remote device and the CO device shall interoperate; if the gain scaling range supported by the remote device is small, the CO device is only able to adjust the ADSL transmitting power level within the gain scaling range supported by the remote device; secondly, chips of many vendors do not support such a gain scaling value as −14.5 dB since the recommended gain adjusting range is −2.5 dB˜+2.5 dB.